Polyoxometalates (POMs) are a unique class of inorganic metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge which is balanced by cations that are external to the cage, and may also contain centrally located heteroatom(s) surrounded by the cage framework. Generally, suitable heteroatoms include Group IIIa-VIa elements such as phosphorus, antimony, silicon and boron. The framework of polyoxometalates comprises a plurality of metal atoms (addenda), which can be the same or different, bonded to oxygen atoms. Due to appropriate cation radius and good π-electron acceptor properties, the framework metal is substantially limited to a few elements including tungsten, molybdenum, vanadium, niobium and tantalum.
In the past, there have been increasing efforts towards the modification of polyoxoanions with various organic and/or transition metal complex moieties with the aim of generating new catalyst systems as well as functional materials with interesting optical, electronic and magnetic properties. In particular, transition metal substituted polyoxometalates (TMSPs) have attracted continuously growing attention as they can be rationally modified on the molecular level including size, shape, charge density, acidity, redox states, stability, solubility, etc.
For example, Angus-Dunne et al. describe the preparation of palladium-susutituted sanwich-type polyxometalate K2Na6[Pd2W10O36].22H2O (see: J. Chem. Soc., Chem. Commun. 1994, 523-524). This polyanion is composed of two W5O186− moieties linked by two palladium(II) ions in square-planar environments.
Lee et al. disclose the structure of several platinum(IV)-substituted polyoxometalates, such as (CH6N3)8[α-SiPt2W10O40].6H2O, a Keggin-type polyanion in which two addenda atoms are replaced by Pt atoms (Acta Crystallographica, Section C, 2003, C59, m152-m155), as well as K2[H6-α-PtMo6O24].5H2O (Acta Crystallographica, Section C, 1994, C50, 1657-1659), (CH6N3)8[PtW6O24] (Acta Crystallographica, Section E, 2003, E59, m116-m118), (NH4)4.5[H3.5-α-PtMo6O24].1.5H2O, (NH4)4[H4-β-PtMo6O24]. 1.5H2O and K3.5[H4.5-α-PtMo6O24].3H2O (Bulletin of the Korean Chemical Society 15, 1994, 37-45), which all have the so-called Anderson structure.
Kortz et al. report on the palladium(II)-substituted, dimeric, lone pair containing polyanion [Cs2Na(H2O)10Pd3(α-SbW9O33)2]9− (Inorg. Chem. 2004, 43, 3915-3920). This polyanion was synthesized by reacting Pd(CH3COO)2 with [α-SbW9O33]9− in aqueous acidic medium. The square-planar palladium(II) ions are located in the central belt of the sandwich-type structure connecting two (α-SbW9O33) Keggin moieties via bonding to oxygen atoms of the WO6 octahedra.
Hill et al., Science 2004, 306, 2074-2077, disclose the synthesis of the Pt(IV)-containing compound K7Na9[Pt(O) (H2O) (PW9O34)2].21.5 H2O. The polyanion [Pt(O) (H2O) (PW9O34)2]16− is composed of two (A-α-PW9O34) Keggin units linked by an octahedral platinum(IV) center with terminal oxo and water ligands. Attempts to prepare the palladium(IV) analogue of this complex proved unsuccessful.
Kortz et al., Inorg. Chem. 2004, 43, 8367-8372, describe the synthesis of a palladium(II)-substituted tungstosilicate, [Cs2K(H2O)7Pd2WO(H2O) (A-α-SiW9O34)2]9−. This polyanion was synthesized by reacting Pd(CH3COO)2 with [A-α-SiW9O34]10− in aqueous acidic medium. It is composed of two (A-α-SiW9O34) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group, two Pd(II) centers, one potassium ion and two cesium ions. The palladium(II) ions exhibit square-planar coordination geometry as they are only ligated to four oxo groups of the polyanion backbone. The potassium and cesium ions are also coordinated by terminal water molecules.
Moreover, Kortz et al. report on the palladium(II)-substituted, lone pair containing polyanion [Cs2Na(H2O)8Pd3(α-AsW9O33)2]9− (Eur. J. Inorg. Chem. 2005, 3034-3041). This polyanion was synthesized by reacting PdCl2 with [α-AsW9O33]9− in aqueous acidic medium. The square-planar palladium(II) ions are located in the central belt of the sandwich-type structure connecting two (α-AsW9O33) Keggin moieties via bonding to oxygen atoms of the WO6 octahedra. This polyanion can be considered as the As-analogue of the above mentioned [Cs2Na (H2O)10Pd3 (α-SbW9O33)2]9−.
Kortz et al. also report on the palladium(II)-substituted, lone pair containing polyanion [Na2(H2O)2PdWO(H2O) (α-AsW9O33)2]10− (Eur. J. Inorg. Chem. 2005, 3034-3041). This polyanion was synthesized by reacting PdCl2 with [AS2W19O67(H2O)]14− in aqueous acidic medium. It is composed of two (α-AsW9O33) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group, a square-planar Pd(II) center and two sodium ions. One of the two sodium ions in the central belt is located in an addenda atom position, which appears to decrease the stability of this polyanion in solution.
Recently, Hill et al. describe the preparation of a Pd(IV) -oxo compound, K10Na3[PdIV(O) (OH)WO(OH2) (PW9O34)2], by reacting PdSO4 with [A-α-PW9O34]9− yielding [PdII3(PW9O34)2]12− which undergoes rapid loss of Pd(II) to form [PdIIWO(OH2) (PW9O34)2]12− and oxidation of this compound resulted in the Pd(IV)-oxo complex (Hill et al., J. Am. Chem. Soc. 2005, 127, 11948-11949). This polyanion is composed of two (A-α-PW9O34) Keggin moieties which are linked via a central belt consisting of a {WO(H2O)}4+ group and a {PdO(OH)}+ group. The tungsten center in the belt has an internal oxo ligand and an external water ligand, whereas the Pd center has an internal oxo and an external hydroxy ligand.
However, up to now the known Pd— and Pt-based anions have not turned out to be very useful for homogeneous or heterogeneous catalytic applications.
Therefore, it is an object of the present invention to provide palladium- and/or platinum-substituted polyoxometalates which are useful as catalyst in homogeneous and heterogeneous oxidation reactions of organic substrates. Furthermore, such Pd— and/or Pt-substituted POMs should be easy and reproducible to prepare.